Notes

General:
All API functions are subject to NT's security system.
Some handles are mapped to internal handles by the emulation layer in order
to remember things that NT doesn't. These mapped handles are stored in handle
tables which does not allow simultaneous access by more that N threads, where N
is a suitably value selected by me.
Most error codes are interpreted but some are passed back directly. Most error
native NT error codes are the same for OS/2 so it will most likely work, but
you may get errors that you would not under OS/2.

Files:
At most ~5 files can be accessed at a time. The emulation layer tries to
release the locks in order to allow DosClose() to abort i/o in other threads
but do not rely on it.
At most 50 files can be open.
Handle 0, 1 and 2 are mapped correctly to NT's STD_INPUT_HANDLE,
STD_OUTPUT_HANDLE and STD_ERROR_HANDLE
EAs are not supported. Attempts to retrieve or set EAs will fail.
DosOpen()
  disk-relative filenames ("D:file.foo") may not work
  If openflags are set to OPEN_ACTION_FAIL_IF_NEW|OPEN_ACTION_FAIL_IF_EXISTS the
  call will fail.
  OPEN_FLAGS_SEQUENTIAL is mapped to NT's FILE_FLAG_SEQUENTIAL_SCAN
  OPEN_FLAGS_RANDOM is mapped to NT's FILE_FLAG_RANDOM_ACCESS
DosQueryHType()
  bit 7 (0x80) are always set because it is not possible to retrieve the
  information under NT, so it is best to assume the worst.
DosSetMaxFH()
  anything but 50 handles fais
DosSetRelMaxFH()
  50 is always returned in *pcbCurMaxFH
DosCancelLockRequest()
  Is only a stub that returns 50 (error_not_supported)
DosFindFirst()
  A maximum of 50 open HDIRs is supported - remember to call DosFindClose()
DosForceDelete()
  is the same as DosDelete()
DosCopy()
  DCPY_APPEND is not supported
DosEditName
  Is only a stub that fails
DosSetDefaultDisk
  NT does not support CWDs on anything but the current disk. CWDs are
  remembered like BC4 (envvars), but DosOpen() and DosFindFirst() does not
  honour them.
DosQueryCurrentDir()
  with disknum=0 (default) a UNC path may be returned
DosQueryVerify()/DosSetVerify()
  verify flag is remembered but ignored
DosSetPathInfo()
  Is only a stub that fails
DosQueryFSInfo
  fails if disknum=0 and CWD is a UNC path
DosSetFSInfo
  fails if disknum=0 and CWD is a UNC path
  volume serial number is not set

Memory:
Nothing is remembered, so access protection etc. is calculated from what NT
tells us. For instance, pages with PAG_GUARD attribute on this attribute is
remembered and PAG_READ/PAG_WRITE... is forgotten
Uncomitted pages does not have any attributes.
Shared memory is supported but at most 64 shared objects system-wide can by
present at a time. Shared memory is allocated from 0x10520000. If this area has
been used by something else shared memory cannot be used.
DosGiveSharedMem()
  Slow compared to native OS/2
DosSubSetMem()
  Shared memory heaps are not supported

DosQuerySysInfo
  Some information is made up.

DLLs
DosLoadModule
  "not_emulated" is returned in pszFailName
DosQueryProcType
  PT_32BIT is always returned in *pulproctype

Named pipes
Nonblocking named pipes are not supported
Named pipe event semaphores are not supported
DosCallNPipe
  Bypasses file handle mapping table
DosCreateNPipe
  Specifying 0 as maxinstances will fail
  Default pipe wait time is 50msec
DosQueryNPHState
  If more that 255 instances exist 255 will be returned
DosQueryNPipeInfo
  infolevels other 1 are not supported
DosQueryNPipeSemState
  Is stub that always fails
DosSetNPipeSem
  Is stub that always fails

Processes/threads
Max 4096 threads are suported
Thread IDs are mapped since NT TIDs cannot be used for anything
DosBeep()
  doesn't
DosExecPgm
  Debugging is not supported
  "not_supported" is always returned in pObjname

Queues
Slow compared to OS/2

Semaphores
Shared mutex semaphores are partially supported. Shared event semaphores are
partially supported. shared muxwait semaphores are not supported.
MuxWait semaphores are handle-mapped. Max 10 MuxWait operations at a time
DosResetEventSem
  Post count is not remembered so only 0 or 1 is returned in *pulPostCount
DosQueryEventSem
  Post count is not remembered so only 0 or 1 is returned in *pulPostCount
DosOpenEventSem
  Always fails for unnamed event semaphores